1. Field of the Invention
The present invention relates to a high-side switch driver, and more particularly to a driver circuit for driving the high-side transistor.
2. Description of the Related Art
A variety of power converters and motor drivers utilize bridge circuits for controlling a power source to a load. The bridge circuit generally has a high-side switch coupled to a power source and a low-side switch coupled to a ground reference. A common node between the high-side switch and the low-side switch is coupled to a load. High-side and low-side switches are generally embodied in transistors. As switches are controlled to alternately conduct, the level at the common node is swung between the power source and the ground reference. Therefore, the level at the common node is shifted to the power source when the high-side transistor is turned on. In order to fully turn on the high-side transistor for achieving lower impedance, a gate driving voltage must be higher than the power source. Therefore the gate-to-source of the high-side transistor must be floated with respect to the ground reference. FIG. 1 illustrates a conventional bridge circuit using a bootstrap capacitor 30 and a charge-pump diode 40 for creating a floating voltage VCC for driving a gate of the high-side transistor 10. When a control transistor 45 is turned on, the gate of the high-side transistor 10 is therefore connected to the ground reference via a diode 42. This turns off the high-side transistor 10. Once the high-side transistor 10 is turned off and a low-side transistor 20 is turned on, the bootstrap capacitor 30 is then charged up by a bias voltage VB via the charge-pump diode 40. The floating voltage VCC is propagated to the gate of the high-side transistor 10 by switching off a control transistor 45 via a transistor 41. The high-side transistor 10 is turned on as a result of the aforementioned switching off of a control transistor 45.
One drawback of the aforementioned bridge circuit is its high switching losses in high-voltage applications. The control transistor 45 requires a high voltage manufacturing process suitable for high-voltage applications (at 200 volts or more). The high-voltage transistors typically have a larger parasitic capacitor, which increases the rising-time and therefore slows down the switching signal. High switching losses from the high-side transistor is resulted from the aforementioned bridge circuit. Therefore, the aforementioned bridge circuit is inadequate for high-voltage and high-speed applications.
Many recently developed bridge circuit designs include methods of generating a suitable gate-voltage for the high-side transistor. Some well-known conventional bridge circuit designs include Zisa et al. (U.S. Pat. No. 5,381,044), Johnson (U.S. Pat. No. 5,638,025), and Nadd (U.S. Pat. No. 5,672,992). The aforementioned bridge circuits share the same drawbacks as the conventional circuit shown in FIG. 1. The control transistors of the conventional bridge circuit designs produce high switching losses in high-voltage applications.
To overcome some the aforementioned deficiencies, a conventional bridge circuit using a boost converter technique was introduced by Milazzo (U.S. Pat. No. 6,344,959). However, the aforementioned technique uses a voltage doubling circuit, which requires an additional switching element and other circuitries; therefore, costs and complexities of the driving circuit are increased. Other conventional high-side transistor drivers for high-speed applications are described in Yang (U.S. Pat. No. 6,781,422) and in Yang (U.S. Pat. No. 6,836,173). However, the higher power consumption remains to be a disadvantage.
The objective of the present invention is to overcome the drawbacks of the conventional bridge circuits and to provide a high-side transistor driver having high efficiency suitable for high-voltage and high-speed applications.